Sciencemadness Discussion Board

Why are rhubarb leaves poisonous but not spinach?

The Plutonium Bunny - 10-7-2021 at 18:50

So I was attempting an extraction of oxalic acid from rhubarb leaves today (as one does), and I came across an interesting Wikipedia article ( that lists the oxalic acid content of various plants. As a side note, I'm extracting the acid as a project for fun, not to obtain anything truly useful. The yields will be tiny.

I was surprised by reading the Wikipedia page saying both spinach and Swiss chard have oxalic acid contents of approximately 1g/100g leaf. Rhubarb leaves, however, have only 0.5g/100g leaf. People eat spinach and Swiss chard all the time, but it is rhubarb leaves I have been told are poisonous.

Why is spinach OK but not rhubarb leaves (serious question)? One would think the higher oxalic acid content would make spinach an even stronger poison than rhubarb leaves.

j_sum1 - 10-7-2021 at 18:56

At a guess I would suggest that one contains the acid and the other an insoluble salt. They would not have the same bioavailability.

unionised - 11-7-2021 at 10:50

The anthraquinones don't help.

The Plutonium Bunny - 11-7-2021 at 11:41

OK, that would make sense - I read about some of the oxalic acid content possibly being calcium oxalate, so that would be pretty insoluble when consumed. I tried adding a good splash of HCl to the extraction water when I heated the ground leaves to account for any calcium oxalate, not sure if that would actually work.

Right now the extraction is a *#$%-awful smelling brown liquid. When I added calcium chloride (aq.) to crash out the oxalate, I got a brown precipitate (!!). Not sure why that happened. Also unsure if oxalic acid would degrade over time, as these are dried leaves from last year.

unionised - 11-7-2021 at 12:04

Calcium oxalate is bad news anyway.
It's soluble in HCl - so your stomach acid would set the oxalate free.


digga - 11-7-2021 at 16:30

I have seen issues with iron contamination in commercial ice melt grade calcium chloride. That may be the source of your brown color.